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Keenaght Water, Cullion TownlandLondonderry
Summary Full report
Site Type: Stream section
Site Status: PASSI
District: Magherafelt District Council
Grid Reference: H8190
Rock Age: Carboniferous (Asbian, Visean)
Rock Name: Armagh Group, Desertmartin Limestone Formation, Leitrim Group, Meenymore Formation
Rock Type: Conglomerate, Limestone, Mudstone, Sand, Sandstone, Septarian nodule
Fossil Groups: Bivalve, Brachiopod, Coral, Nautiloid
Other interest: No data, Fluvial sediments, Marine sediments, sabkha, tidal flat

Summary of site:

This stream section is important for two reasons. It is the most northerly occurrence of Carboniferous rocks of the Asbian stage in Northern Ireland (rocks around 338 million years old) and the only known occurrence (and the most easterly) of the Meenymore Formation outside Fermanagh and Tyrone.

Both outcrops occur in the stream of the Keenaght Water draining north east from the saddle between the main mass of Slieve Gallion and the subsidiary summit to the north of Crocknamohil. Only 2 m of strata are exposed in the northern part of the section, about 200 m upstream from the road. The lower beds are conglomerates and pebbly sandstones with larger pebbles and cobbles formed from white vein quartz. At the top are two limestones incorporating grains of vein quartz containing a fauna of shelly fossils including the coral Lithostrotion portlocki and four lamp shell (brachiopod) species of which Gigantoproductus, probably of the species semiglobosus, is especially significant. The coral and this species suggest an Asbian, possibly late Asbian, age. 350 and 500 m upstream from this small outcrop more extensive exposures of rocks of the Meenymore Formation are present taking typical form. 5 m of dark grey, richly fossiliferous, laminated (in fine layers) mudstones are sandwiched between brown medium-grained sandstones and barren mudstones. Although the fossils are plentiful, mostly brachiopods, small solitary corals, bivalves and conical nautiloids, they form a fauna restricted in species. This suggests some limitations imposed by environmental conditions. The very confined exposure of the Desertmartin Limestone makes interpretation difficult but its coarse quartz nature suggests shallow water with energetic flow almost certainly inshore close to the mouth of a river disgorging coarse land debris. The total thickness here is unknown but it is thought that these are not the basal beds of the formation. The fossils in the Meenymore rocks are all marine in origin marking out a period of open marine conditions but the enclosing rock suggest shallow tidal waters at times with periods of highly saline, inshore conditions with extensive, salt-encrusted coastal flats. This outcrop massively extends the known area of the Meenymore Formation now thought to have covered over 5,000 square kilometres in a belt extending from County Sligo through the Fermanagh Highlands, the Clogher Valley and onto Slieve Beagh and now, confirmed by this locality, reaching the Desertmartin area. This baking environment, hostile to life, occurred along the coast of the supercontinent of Laurentia as it drifted northwards towards the equator almost 340 million years ago. The presence of the Meenymore Formation in this area has economic implications, particularly if it extends beneath the Antrim Basalts to the east. The formation has proved to be the richest source of hydrocarbons in the Carboniferous succession and the Antrim Basalts could provide an excellent cap to suitable oil and/or gas reservoir structures.

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